1. Field of the Invention
The invention relates to an opaque, multilayer polypropylene film which comprises at least one base layer containing polypropylene or a polypropylene mixture and fillers, having at least one interlayer and at least one outer layer applied to this interlayer.
The film according to the invention is distinguished by a characteristic matte appearance.
The invention furthermore relates to a process for the production of the multilayer film and to products made from the film.
2. Description of Related Art
German Patent Application P 41 35 096.0 discloses multilayer polyolefin films comprising a polypropylene base layer and an outer layer made from an HDPE blend. This HDPE blend comprises HDPE and copolymers and/or terpolymers made from .alpha.-olefins and, if desired, polypropylene. The outer layer of the multilayer film has minimum sheen and maximum haze, giving the film a characteristic matte appearance.
DE-A-16 94 694 discloses multilayer films which comprise biaxially oriented polypropylene films and at least one heat-sealable layer made from an ethylene-propylene copolymer. This films have good heat-sealability and are clear. However, the films have inadequate scratch resistance and unsatisfactory processing properties in high-speed packaging machines.
EP-A-0 008 904 discloses a biaxially oriented three-layer polyolefin film which is heat-sealable on both sides and in which the base layer is formed from propylene polymers and the two heat-sealable layers are formed from heat-sealable olefin polymers. However, this polyolefin film is transparent and, in particular, has only low scratch resistance and is not printable. In addition, its sliding properties are unsatisfactory in high-speed packaging machines.
A further biaxially oriented three-layer polyolefin film which is heat-sealable on both sides is disclosed in U.S. Pat. No. 4,419,411. The two heat-sealable layers in these films incorporate polysiloxane and silicon dioxide as an additive combination. The base layer essentially comprises polypropylene and contains a small amount of a monocarboxamide, which partially migrates from the base layer into the two heat-sealable layers. The multilayer polyolefin film disclosed is said to have a particularly low coefficient of friction. The disadvantage of the film is that it is not printable.
U.S. Pat. No. 4,578,316 discloses a biaxially oriented multilayer polyolefin film whose base layer comprises a polypropylene homopolymer and whose outer layer comprises a blend of polypropylene and MDPE and/or HDPE. This polyolefin film has a low coefficient of friction together with a readily wettable surface; however, the disadvantage of this film is that it has excellent optical transparency.
WO 89/10839 describes multilayer polyolefin films whose base layer comprises a polypropylene polymer and whose outer layers comprise either HDPE or an ethylene-propylene copolymer or a polypropylene, the film being stretched under particular temperature conditions. It has, in particular, a low coefficient of friction and good wettability, but also has the disadvantage of high sheen and low haze.
EP-A-0 367 613 discloses a multilayer film comprising a vacuole-containing polypropylene base layer and an inscribable outer layer which contains a first polymer having a melt-flow index of .ltoreq.1 g/10 min and a second polymer which is incompatible with said first polymer, with incompatible meaning that the two polymers are present as separate phases. An example given of the first polymer is an HDPE having a density of from 0.92 to 0.97 g/cm.sup.3. The incompatible polymer is a polypropylene or a copolymer of propylene or a terpolymer of propylene. The film described has an opaque appearance, i.e., it is essentially non-transparent to light. It is also disclosed that the film acquires a matte appearance if it is provided with a print. Additionally, it is preferred to incorporate a filler such as SiO.sub.2 into the outer layer to improve the inscribability. The SiO.sub.2 -containing outer layer is also very matte, which gives the film a paper-like appearance. The film described has a density of 0.69 g/cm.sup.3. The disadvantage is that the matte, SiO.sub.2 -containing surface proves to be unsatisfactory on printing. This is due to foggy color which does not have a uniform appearance. The foggy color is attributed to inhomogeneities which, due to their opacity, do not appear particularly strongly in the film itself, but come out during printing. The printed films are spotty and are not accepted by a film processor.
Many known films have the disadvantage of chalking, i.e., the films become covered after a few days by a layer of dust, comprising particles not originating from the environment, but from the film itself, in particular from the films outer layers.
All known films are unsatisfactory with respect to their economic advantages and their possible variations. The appearance of conventional films can be varied by suitable measures; for example, the whiteness of a film can be increased by incorporating TiO.sub.2 into the outer layer. However, such modifications generally do not result only in the desired change, but inevitably affect other film properties. For example, TiO.sub.2 in the outer layer changes not only the whiteness but also the roughness and thus the running properties of the film. These side-effects, some of which are very undesirable, result in the processor having to re-optimize the processing conditions for each film type, i.e., resetting packaging machines individually or modifying printing ink compositions, etc. Such necessary measures have previously been regarded as matter of course and unavoidable.